The present invention relates to an optical fiber, particularly an optical fiber capable of standing use in the high-temperature and high-humidity environments, and a fiber-optic sensing system using such an optical fiber.
Optical fiber, in practical use thereof, is usually provided with a protective coat for such purposes as prevention of failure and improvement of handling qualities of the fiber. For instance, fused silica optical fiber produced from fused silica is subject to easy failure for a small crack if it is left uncoated, but secure coating of the fiber in its manufacturing process allows its wide-ranging use for communication. UV-curable acrylic resins and some other types of resins are known as effective coating materials for the optical fiber. In some applications, different types of resins are coated in layers on the fiber. Generally, polymer coating, in virtue of its moderate softness, is capable of mitigating the influence of pressure exerted to the side of the optical fiber to reduce mechanical damage to the optical fiber main body.
On the other hand, the field of application of the currently available optical fiber is not confined to use in the ordinary environments; request is rising for utilization of the optical fiber in more severe environments. Such demand for extended utilization of the optical fiber is notable not only in the field of communication but equally in the fields where it is applied as a sensor. In such applications, the coating material used for optical fiber will also be required to have specific properties malleable to the ambience of use.
For instance, in applications where the optical fiber is destined to be used in a high-temperature environment for a long time, polyimide resin obtained from pyromellitic dianhydrde, known as a typical coating material having heat resistance, is used for coating in a thickness of 25-300 xcexcm (JP-A-1-173006). Since polyimdie resin can withstand heat of up to about 300xc2x0 C., it can well be applied as a heat-resistant material in all the fields excepting certain very specific applications. The polyimide resin, however, because of its chemical structure shown below, has a problem in that when used in a high-temperature and remarkably high-humidity environment for a long period of time, cracks may develop in the coat to make it unable to protect the optical fiber or give damage to the optical fiber main body to cause failure of the fiber: 
wherein R3 is a divalent organic group having 2 or more carbon atoms, and m is an integer of 1 or greater.
The optical fiber coated with said polyimide resin involves a problem in that adhesion between the polyimide coat and the optical fiber is hard to secure. Therefore, long-time use of such polyimide-coated optical fiber in an environment of high temperature and remarkably high humidity may cause condensation of moisture in the spaces between the polyimide coat and the optical fiber. Particularly in the case of fused silica optical fiber, presence of such moisture encourages growth of the microcracks in the fiber surface, which may end up with failure of the optical fiber.
An object of the present invention is to solve the foregoing problems of the prior art and to provide an optical fiber capable of standing long-time use even under remarkably severe conditions of high-temperature and high-humidity previously not applied, and a fiber-optic sensing system with high sensitivity and reliability using such an optical fiber.
The present invention provides an optical fiber comprising an optical fiber main body and a coat formed therearound, said coat being made from a polyimide resin obtained by using as its acid component a tetracarboxylic dianhydride in which at least two benzene rings are fused or bound directly or via an oxygen atom.
The present invention is also envisaged to provide a fiber-optic sensing system comprising signal transmitting paths for transmitting input and output light signals and made from an optical fiber, and at least one sensing part, wherein the input light signal is changed according to the object to be measured to issue an output light signal, said optical fiber in said signal transmitting path having a protective layer thereon, said sensing part being constructed using an optical fiber having no protecting layer, and at least said optical fiber in the sensing part being the one coated with said polyimide resin.